Import by categories created a duplicate category for every post in that category, while dropping others. I had xwy-category-2 through xwy-category-295 to deal with and a quick database search and replace doesn't work as I couldn't get a wildcard search and replace to work across all 295 categories, but might soon have a more complicated solution to resolve this - which shouldn't happen in the first place.
Category exports and imports bring in ALL of the categories and tags for the entire blog, not just the ones being imported. Breaking a site up into subdomains (categorized) meant cleaning up all the unwanted and unnecessary categories and tags including the duplicates.
Category exports with more than one category seem to only honor one of the two categories and not the second one, exporting and importing only some of the posts but not always consistently.
Removal from the core was a mistake, I feel, but this is a disaster of time wasting proportions. The import, which should have taken at most an hour with all the categories, maybe less, has taken over a week to clean up and redo over and over again.
We are now sticking with database exports.